Today I want to talk about sleep paralysis, but first I want to talk about cats (because it’s the internet and all). Have you ever grabbed a cat by the loose skin on the back of their neck and watched them go rigid. This physiological response is vital to many aspects of the feline life cycle from mother cats maintaining control of kittens during relocation to preventing dangerous struggle during mating.

Humans also have a similar mechanism of tonic immobility, but ours isn’t quiet so accessible to the outside world. When we dream, our brains create some wild and often violent scenarios. If our bodies were allowed to react to these scenarios, we would be very dangerous indeed! Kicking and thrashing, punching and…snacking? Yes, there is a parasomnia in which people leave their bed in a dream state, eat and return to bed with no memory of its happening. Ideally, the body safeguards from this behavior by creating a damper to outgoing neural activity by halting the release of monoamines (norepinephrine, serotonin and histamine, which I will discuss at a later date). Your brain can stay active for dreaming and processing information while you remain safely in bed (and safe from predators on an evolutionary level). This is known as REM atonia.

Sleep paralysis is somewhat opposite of sleepwalking. This is what happens when that mechanism remains active while the person is becoming aware of his or her surroundings. The brain is still producing dreamlike brain waves, so the mechanism stays in place to keep the body still. The person will be able to see, hear and feel everything around them as if they are awake, but their mind will still be incorporating imagery from dreams. At this point a person might see some horrifying monster descending upon them and be completely paralyzed. They will be unable to call out for help, even though they feel completely alert. This is where we get the concept of the incubus. Some instances are very short and infrequent (isolated sleep paralysis, more commonly experiencing the incubus scenario), while others can last for up to 30 minutes multiple times in a person’s life (recurrent isolated sleep paralysis, more commonly experiencing out-of-body scenarios). The latter can even happen multiple times the same night. Hypnagogic sleep paralysis occurs as a person is falling asleep. Hypnopompic sleep paralysis occurs as one is waking. Both terms originate from the Greek word “hypnos” meaning sleep, and the words “agogos” meaning leading, and “pompe” meaning sending.

There are three types of hallucinations a person may experience during sleep paralysis, one I’ve mentioned already is the incubus, but I will get back to that in a moment. The first type is the intruder. Because of the increased activity in the amygdala during REM sleep, the brain is quick to assess threat. You can blame the human fear of the unknown on evolution as our brains are wired to perceive ambiguous stimuli as dangerous. We’re programmed for caution. If the sleeper finds some strange apparition and experiences the inability to move or call out, he or she might feel vulnerable to attack and panic.

The incubus adds upon this reaction. Panic affects your breathing, but during sleep paralysis, you still have no voluntary control over your breathing which is still very shallow (or in cases of sleep apnea, there is an actual blockage). In these instances, the dreamer may perceive that the intruder is strangling him or sitting on his chest.

Finally, there is the vestibular-motor hallucination, or out-of-body experience. This is not threat related, but instead the brain believes it is receiving external stimuli from movement where none is truly occurring. This is the type commonly experienced in recurring isolated sleep paralysis.

When diagnosing sleep paralysis, doctors commonly check for narcolepsy due to its high co-morbidity and availability of a genetic test. These are heritable disorders, and it is very common if occurring in one identical twin it will be present in the other. Lifestyle indicators also increase the risk of sleep paralysis. If you aren’t getting plenty of rest or aren’t keeping a regular sleep schedule, this could sometimes lead to the overlap of sleep phases when you are able to sleep. Sleeping on your back, which is sometimes recommended for treatment of other disorders, can also leave you susceptible to sleep paralysis, especially the incubus type. Other factors include any type of activity or circumstance that leaves your body exhausted.

There is no way to completely cure sleep paralysis, though it is possible to decrease the frequency through the same means I described earlier for increasing the duration of slow-wave sleep (that is via use of SSRIs). While terrifying, sleep paralysis is not considered dangerous. Narcolepsy, on the other hand, is dangerous, and that treatment takes priority.

Sleep paralysis comes with rich folklore from every part of the world. Demons, witches, shadow people and spirits frequent these tales, and it is also related to the superstition that cats will steal your breath while you sleep. How creative and horrifying the dreaming mind can be!